Sintered ni-al-zr compositions



2,884,688 SINTERED Ni-Al-Zr COMPOSITIONS William H. Herz, Yonkers, N.Y., assignor to Borolite Corporation, Pittsburgh, Pa, a corporation of Delaware No Drawing. Application December 28, 1956 Serial No. 631,074 2 Ciaims. (Cl. 2)--182) This invention relates to an alloy composition having high temperature oxidation resistance and high temperature stress-to-rupture resistance, and the process for making the same.

The composition of this invention is especially desirable for service at elevated temperatures and is particularly applicable for the manufacture of parts which are subjected to corrosive fluids especially where such parts are to be subjected to corroding conditions within a corroding atmosphere at high temperatures.

in the search for high-temperature materials for such application, numerous Ni-Al alloys have been tried, but without achieving satisfactory results. Among the principal literature references describing such Ni-Al alloys, are the articles of H. V. Kinsey and M. T. Stewart (A Nickel-A111minum-Molybdenum Creep Resistant Alloy, Canadian Journal of Research, vol. 27, sec. F, pp. 80-98; "Nickel-Aluminum-Molybdenum Alloys for Service at Elevated Temperatures, Transactions of the ASM, vol. 43 (1950) pp. 193-225), wherein they state that Ni-Al alloys containing a phase identified as NiAl, are unsuited for the manufacture of gas turbine blades. On page 197 of the ASM Transactions article, for example, they say, The usefulness of the alloy disappears when the ratio of nickel to aluminum becomes 6:1 or less. This deterioration of properties coincides with the appearance of a phasetFig. 15) identified as NiAl, and again, on' page 225. of the same article, it is stated, We have produced test bars consisting completely of NiAl. These bars would literally fall apart on handling, apparently from lack of cohesion between the grains.

Among the objects of the invention is a high-temperature material or shaped body combining as a principal ingredient a nickel-aluminum alloy or the intermetallic compound NiAl, but which does not deteriorate and which retains a high transverse rupture strength for a 4 prolonged period at temperatures of about 1000 C. and higher.

The present invention is based on the discovery that by forming a shaped body by a powder metallurgy process out of particles of a composition containing essentially the intermetallic compound NiAl and an addition of 2% to 7% zirconium, the resulting body may be given by powder metallurgy'techniques 100% density, and it will retain high corrosion resistance and the desired high transverse rupture strength not only at temperatures in the'range up to 1000 C. but even at temperatures higher than 1000 C., such as 1100 C. to 1200f C. and even higher.

The foregoing and other objects of the invention will be best understood from the following description of exemplifications thereof.

The intermetallic compound NiAl consists of about 68.5% nickel and 31.5% aluminum, but small variations on both sides of the exact'stoichiometrical proportion will not materially aifect its properties, a larger variation] being permissible; on the side of excess nickel so that the most desirable, range of proportions from the standpoint of oxidation resistance corresponds to an Ni/Al ratio varying-from 65 :35 to 75 :25. f the known NiAlcompounds, the highest oxidation'resistance is exhibited by NiAl containing about excessnickel.

Z,884,d88 Patented May 5, 1959 According to the invention, shaped bodies of the same high corrosion resistance as the best prior NiAl materials, but having a higher order of transverse rupture strength at elevated temperatures, are obtained by forming the shaped bodies with powder metallurgy methods out of particles of Ni and Al generally corresponding to the NiAl compound, and containing in addition, 2% to 7% Zr, and treating the ingredients to homogenize them into an NiAl alloy containing in solid solution the Zr content. Particularly desirable bodies of great corrosion resistance at elevated temperatures and high transverse rupture strength in the range up to 1000 C. and higher, are obtained by forming them out of metal particles corresponding generally to the compound NiAl, with an addition of 3% to 6% Zr. In general, it is desirable that the Zr addition should be of high purity. Good results are obtained by adding the zirconium in the form of zirconium hydride. In practice, the powder particles for the shaped body are advantageously first produced out of a mixture of nickelpowder, aluminum powder and zirconium hydridepowder, which mixture is heated at an elevated temperature until an exothermic reaction starts, which i'urther raises the temperature of the mixture to'the temperature at which the ingredients of the mixture are formed into a porous, homogeneous, conglomerate containing Ni, Al and Zr in proportions corresponding to the ingredients of the mixture. The so-obtained porous conglomerate is reduced to a powder by crushing and milling to a fine particle size such as 325 mesh. The resulting fine NiAl-l-Zr powder is then formed into thedesired shaped product by coldpressing or compacting, presintering, followed by final sintering, or by cold-pressing followed by sinteri ng, or by hot-pressing. The resulting fine NiAl+Zr may also be formed into shaped bodies by hydrostatic pressing or slip-casting, followed by presintering, final shaping and final sintering.

A feature of the invention is the fact that a compacted body of homogeneous powder particles of the formula generally corresponding to NiAl containing 2% to 7% Zr in solid solution densified to density and great high-temperature strength within a sintering time of only about five minutes. Comp'acted or cold-pressed NiAl+Zr powder particles may be given the desired 100% density by sintering at temperatures between 1500 C. and 1600 C. in vacuum or under a protective atmosphere of hydrogen or an inert gas such as argon for 2 to 5 minutes When the NiAl-l-Zr powder particles are formed into shaped bodies by hot-pressing, the hot-pressed powder body is heated to aboutl325 C.1425 C. for about ninety seconds to give it 100% density.

The heatingof the powder mixture of the nickel powder, aluminum powder and zirconium or zirconium hydride powder, is done in an enclosure or crucible of a material which does not react with the ingredients of the metal powder mixture when they undergo an em, thermic reaction in forming the zirconium-containing NiAl. Good results are obtained by using as a crucible enclosure for the powder mixture of Ni, Al and Zr particles, a crucible or enclosure of magnesia. It is good practice to place around the magnesia crucible enclosure containing the powder mixture of Ni, Al and Zr, an outer enclosure of graphite.

Before treating to the temperature of the exothermic reaction, his good practice to compact the powder mixture of Ni, Al and Zr into solid slugs of a size convenient for charging the crucibleenclosure. In practice, good resuhsare obtained by compacting the powder mixture of Ni, Al and irconium hydride particles into slugs about two inches in diameter and two inches high, having a density of. about 3 gm./cc. (gram/cubic centimeters).

The shape, size and densityof the compacted powder slugs, are not important as long as the metal powder mixture is retained in the form of solid pieces suitable for ready charging into the interior of the crucible enclosure in which they are subjected to the exothermic reaction. To prevent binding, the walls of the compacting die and compacting punches are suitably lubricated, for instance with a camphor-ether solution. 7

The crucible enclosure in which the powder particles of Ni, Al and Zr are heated for bringing about the exothermic reaction and homogenization of the ingredients, is provided with an inlet duct and an outlet duct for admitting hydrogen to the interior of the crucible into contact with the treated powder mixture and for discharging the hydrogen from the interior of the heated crucible enclosure. It is of advantage to use purified dry hydrogen. This can be done by any conventional procedure, for instance, by passing the hydrogen through a palladium catalyst followed by passing it through a desiccant tower, for instance one containing activated alumina. It is good practice to purge the system with pure hydrogen before raising the temperature of the contents from normal temperature to the reacting temperature.

The heating of the Ni, Al and zirconium hydride powder mixture to the reaction temperature may be effected by high-frequency induction heating, for instance, by surrounding the crucible enclosure with its powder mixture content with a water-cooled copper coil through which high-frequency electric current is fed, for heating the crucible contents by induced electric currents. The hydrogen in the interior of the crucible is ignited as the metal powder charge is heated by the induced currents. Depending on the amount of the metal powder charge, a longer or shorter time is required for bringing the pow der mixture charge of the crucible to the reaction temperature between 800 C. and 1200 C. The exothermic reaction raises the temperature of the heated powder mixture to a higher temperature, such as 1650" C. Purified hydrogen is continuously passed through the interior of the heated crucible as the charge is being heated to start the exothermic reaction, and the passing of the hydrogen is continued until the reaction is completed and the ingredients of the powder mixture charge are homogenized into NiAl containing the small addition of zirconium in solid solution therewith. As an example, a powder charge of Ni, Al and Zr, in the above-specified proportions, filling a cylindrical crucible interior 4 inches inside diameter and 17 inches high, treated in the manner described above, will be heated by induction to the exothermic reaction temperature within about 20 minutes. and a homogeneous conglomerate of the reaction product is obtained by continuing the treatment for about 90 min utes at an elevated temperature of 1200 C. under an atmosphere of pure hydrogen. The purified hydrogen is passed continuously through the interior of the crucible until after the completion of the exothermic reaction and the homogenizing of the contents of the reacted powder mixture, the temperature of the reaction product is lowered to normal temperature. The combined exothermic and homogenizing treatment carried on at the elevated temperature yields a porous but tough conglomerate cntaining NiAl with the small addition of zirconium in solid solution therewith. The so-obtained conglomerate is removed from the crucible and blasted with grit or sand to remove any surface contaminants. The conglomerate is then crushed into a powder, such 100 mesh particle size. and the powder body is further reduced in size, as by ball-milling in a tungsten carbide mill under a cover of a suitable protective liquid such as ethyl alcohol, until the particles of the conglomerate have been reduced to a size of 2 to microns, or 325 mesh particle size. After the treated conglomerate has thus been reduced to the fine particle size, the powder particles are suitably separated from the cover liquid, such as ethyl alcohol, as by filtering and drying.

Out of the so-obtained powder mixture of fine, homogenized particles of NiAl containing Zr in solid solution, shaped bodies may be produced either by cold-pressing or compacting, followed by sintering; or cold-pressing followed by presintering at a relatively low temperature, followed by a final shaping or coining, followed by a short final sintering at a higher temperature; or by hotpressing the powder mixture. Good results are obtained by cold-pressing or compacting the powder mixture w1t h 15,000 to 20,000 p.s.i. (pounds per square inch). It is of advantage to use a small amount of lubricant addition with the metal powder mixture when it is so compacted. Any of the known liquid lubricants used in similar compacting processes may be used. Good results are obtained by adding 2% camphor dissolved in ether, to the homogenized metal powder mixture under a fume hood, and the powder body should be compacted relatively soon after admixture of the lubricant, for instance within a day therefrom. It is of advantage to apply a lubncant to the surfaces of the compacting dies and punches, for instance by applying thereto a coating of a similar ethercamphor solution.

Such compacted green body of zirconium-containing NiAl does not have sufi'icient strength for machining into the final accurate shape. If machining or final shaping is to be done, the compacted zirconium-containing NiAl powder body is subjected to presintering. Good results are obtained by presintering at a temperature of between 1000 C. and 1100 C. for about 10 to 20 minutes under vacuum, followed by cooling to about 150 C. before the vacuum is broken. The so presintered body may be readily machined or coined to give it the final desired shape. Instead of presintering under vacuum, the presintering operation may be carried on under an atmosphere of purified dry hydrogen.

The so-co-mpacted and presintered and finally shaped body is then subjected to final sintering. The final sintering may be done under vacuum or in an atmosphere of purified dry hydrogen at a temperature of between 1500 C. and 1600 C. Final sintering for 2 to 5 minutes at temperatures between 1500 C. and 1600 C. results in complete densification of the compact to full or density. Since the final sintering and densification results in an inner shrinkage of about 20%, the compacting dies in which the shaped bodies are formed are dimensioned to give over-sized compacts which after the presintering, machining or coining, and final sintering to full densification and shrinkage, yield a shaped body of the desired final dimensions.

Instead of presintering followed by coining or machining, followed by final sintering, the desired shaped bodies may be produced by subjecting the over-sized compacted zirconium-containing NiAl bodies to a single sintering operation at between 1500 C. and 1600 C. in vacuum or under pure hydrogen for about 3 to 5 minutes up to 10 minutes, whereby the compact is sintered to full density and full strength.

Hydrostatic pressing may be used for forming a zirconium-containing NiAl powder body into the desired compacted shape. For instance, in forming such compact by hydrostatic pressure, the following procedure may be used: A powder body of zirconium-containing NiAl is placed in a rubber mold of desired shape the walls of which will yield when placed in a vessel containing a liquid pressure body which is subjected to pressure. The liquid pressure body may, for instance, consist of glycerin. The metal powder body has admixed thereto, a small amount of a binder such as 2% camphor dissolved in ether. The rubber mold filled with the metal powder body is sealed off and is placed in the pressure liquid, the pressure of which is increased, as by a hydraulic press, for instance, to A to 4 t.s.i. (tons per square inch). Such hydrostatically pressed metal powder bodies may be presintered and further achined before final sintering, or subjected directly to ass gese a final sintering operation, in the same way as described in connection with shaped bodies formed with a compacting die.

Zirconium-containing NiAl powder bodies may also be formed in the desired shape by slip-casting. To this end, the metal powder bodies have admixed thereto a suitable liquid such as water containing a polyvinyl alcohol resin for forming a thick metal powder slurry. The metal powder slurry is then poured into a plaster mold which is vibrated while the slurry is poured into it. After the slurry Contents of the plaster mold have been dried by driving oif the liquid, the shaped cast body is removed from the plaster mold and treated in the same way as the compacted metal powder bodies formed in metal dies.

The examples given below give additional details of procedures suitable for producing shaped bodies of NiAl containing in solid solution 2% to 7% Zr.

Example 1 Nickel powder of high purity, such as electrolytic nickel powder of 99.25% purity, having a particle size -100, +200 mesh, is mixed with aluminum powder of 99.5% purity, -30 mesh particle size, and with zirconium hydride powder, -100 mesh particle size, in pro portions corresponding to a composition containing about 65.8% nickel, 30.2% aluminum, and 4% zirconium. The powder mixture is mixed by tumbling for 15 to 20 minutes, and compressed into slugs approximately 2 inches in diameter, one inch long. The slugs are placed in a crucible of magnesia which is enclosed by a mag nesia closure cover, the covered magnesia crucible being in turn enclosed in a graphite enclosure. Purified dry hydrogen is passed through the interior of the crucible enclosure. The crucible enclosure and the powder content are heated by high-frequency induction currents, until an exothermic reaction takes place at about 1180 C. Purified dry hydrogen is passed through the interior of the crucible enclosure throughout the period during which the contents of the crucible enclosure are heated above normal temperatures and until they are cooled to normal temperatures. Thereupon the reaction conglomerate is removed from the crucible. The reaction conglomerate is porous and is reduced to powder by first crushing it, followed by milling, as in a tungsten carbide ball mill, to form out of the reaction conglomerate, powder particles of --325 mesh particle size. The ball-milling is done under ethyl alcohol, and the resultant NiAl-l-Zr powder is separated from the ethyl alcohol by filtering and drying under vacuum.

X-ray difiraction patterns of so-obtained powder show essentially the presence of NiAl with a greatly expanded lattice indicating that the zirconium content is taken into the NiAl lattice. A chemical analysis shows that the powder particles contain Ni, Al and Zr in proportions corresponding to the ingredients of the initial powder mixture.

A body of such zirconium-containing NiAl powder was mixed with 2% camphor dissolved in other as a lubricant, and compacted in a die with a pressure of 17,000 p.s.i. The compact was subjected to sintering in an atmosphere of purified hydrogen at between 1530 C. and 1540 C. for 5 minutes, yielding a body having a density of 5.85 gm./ cc. (grams over cubic centimeters), this being full 100% density. When subjected to transverse rupture tests, this material had a transverse rupture strength of 138,000 p.s.i. over a temperature range from zero up to 1000 C., and its transverse rupture strength increased above this value as the temperature was increased above 1000 C. Thus, at 1100 C. the transverse rupture strength of the material was 145,000 p.s.i. In contrast, the best prior similarly produced NiAl material exhibiting similar high corrosion resistance and consisting of NiAl plus an excess of 5% Ni in solid solution, although exhibiting a similar high transverse rupture strength at room temperatures, dropped rapidly in strength from 95,000 p.s.i. at 800 C. to 67,000 p.s.i. at 1000 C., and to 50,000 p.s.i. at 1100 C. Likewise in contrast, similarly produced NiAl material containing instead of 4% zirconium, 4% titanium, although exhibiting substantially the same transverse rupture strength up to 1000 C., rapidly lost its strength at above 1000 C., its transverse rupture strength dropping from 140,000 p.s.i. at 1000 C. to 80,000 p.s.i. at 1100 C.

Example 2 A sintered body of zirconium-containing NiAl was produced in the same way as in Example 1, except that the powder mixture out of which the homogeneous conglomerate was formed, consisted of a mixture of powder particles of Ni, Al and ZrH corresponding to a composition consisting of NiAl containing 5% of excess nickel beyond the amount corresponding to NiAl, and 4% zirconium. A homogeneous powder mixture produced by the exothermic reaction treatment as described in connection with Example 1, when compacted and sintered as therein described, yielded a body of 100% density within five minutes of the sintering operation. The resulting sintered body had substantially the same transverse rupture characteristics as the body of Example 1.

Example 3 A sintered body prepared as in Example 1 was formed of an initial powder mixture of Ni, Al, and Zr, corresponding to a composition consisting of NiAl and 2% zirconium. A homogeneous powder mixture produced by the exothermic reaction treatment as described in connection with Example 1, when compacted and sintered as therein described, yielded a body of 100% density within 5 minutes of the sintering operation. The resulting sintered body had substantially the same transverse rupture characteristics as the body of Example 1.

An outstanding feature of shaped bodies of the invention, is the fact that in addition to exhibiting high corrosion resistance and high transverse rupture strength at temperatures above 1000 C., they also have a higher order of impact strength than other known NiAl bodies, a factor of critical importance in the practical application of such shaped bodies.

It will be apparent to those skilled in the art that the novel principles of the invention disclosed herein in connection with specific exemplifications thereof, will suggest various other modifications and applications of the same. It is accordingly desired that the present invention shall not be limited to the specific exemplifications shown or described therein.

I claim:

1. A shaped corrosion-resistant body having an ex tended surface which in operation is exposed to corroding fluids and great strains, said body consisting of sintered powder particles containing about 93% to 98% of a nickel-aluminum alloy of the approximate composition of the NiAl and an addition consisting of 2% to 7% zirconium in solid solution with said alloy, said nickel-aluminum alloy consisting of 65% to 75% nickel, the balance aluminum.

2. A shaped corrosion-resistant body having an ex tended surface which in operation is exposed to corroding fluids and great strains, said body consisting of sintered powder particles containing about 95% to 97% of a nickel-aluminum alloy of the approximate composition NiAl and an addition consisting of 3% to 5% zirconium in solid solution with said alloy.

References Cited in the file of this patent UNITED STATES PATENTS 2,096,252 Koehring Oct. 19, 1937 2,193,435 Smith Mar. 12, 1940 2,491,866 Kurtz Sept. 30, 1942 2,620,555 Lenz Dec. 9, 1952 

1. A SHAPED CORROSION-RESISTANT BODY HAVING AN EXTENDED SURFACE WHICH IN OPERATION IS EXPOSED TO CORRODING FLUIDS AND GREAT STRAINS, SAID BODY CONSISTING OF SINTERED POWDER PARTICLES CONTAINING ABOUT 93% TO 98% OF A NICKEL-ALUMINUM ALLOY OF THE APPROXIMATE COMPOSITION OF THE NIAL AND AN ADDITION CONSISTING OF 2% TO 7% ZIRCONIUM IN SOLID SOLUTION WITH SAID ALLOY, SAID NICKEL-ALUMINUM ALLOY CONSISTING OF 65% TO 75% NICKEL, THE BALANCE ALUMINUM. 